I really like the concept of unlockable characters when done in an interesting fashion like, imo, the first three Smash games. Each having a specific unique condition, ideally somewhat tied to the character themselves like unlocking Luigi via the timer having a 2, or a thematically relevant event match. I miss the challenge of working towards the tougher unlockable characters, the excitement of their screen popping up, and the reward of finally getting them.
Ultimate's felt hollow and perfunctory - and by nature of the quantity, lost the more individualized conditions. Sort of like target test. When that was a thing. Though it was cool what they did with Sephiroth, even though they, understandably, didn't fully commit to it.
Personally, I would not be mad if Smash brought the old style back.
However, I agree that I don't think it's where the genre is anymore, and though I enjoy it, I think for what Smash is, who its audience is, and what people play the game for, having the characters available at the start is more worthwhile.
Perhaps there could be some sort of middle-ground, where maybe all the original characters could be available from the start, but the clones/echoes need to be unlocked. They are somewhat, in effect, "bonus" characters, and that way the underlying moveset (and more recognizable character) is available from the jump.
But in absence of that, keeping some stages locked is something. I think there'd be far less pushback to that. And of course, the mountain of collectibles which just seem intrinsic to Sakurai's design style will always leave those who enjoy the pursuit of unlocking things plenty to chew on, including at the highest difficulties no characters should be partitioned behind.